Method for operating a switch with a connectable current limiter and corresponding arrangement

ABSTRACT

Methods for operating a power switch with at least one main current path and associated secondary current path with an arc extinguisher are known from the art. A current limiter can be used which switches from non-current limiting operation to the current limiting operation at high current loads. The invention is characterized by measures for returning the current limiter during the transition from the current limiting operation to the non-current limiting operation. Specifically power switches are provided with measures for preventing a recommutation of the arc from the secondary current path to the main current path and/or measures for returning the current limiter during the recommutation of the light art. A corresponding arrangement for carrying out the inventive method is provided with means ( 30  to  50 ) in the power switch ( 20 ) and/or current limiter ( 1 ) for preventing a switch failure of the current limiter ( 1 ) disposed in the secondary current path of the power switch ( 20 ). The current limiter can be especially a PTC current limiter ( 1 ).

[0001] The invention relates to a method for operation of a switchingdevice having a switchable current limiter, as claimed in theprecharacterizing clause of patent claim 1. In this case, the expressionswitching device means, in particular, circuit breakers or contactors,and possibly semiconductor switches or the like, as well. In addition,the invention also relates to an arrangement for carrying out the methodas claimed in the precharacterizing clause of patent claim 7.

[0002] Switching devices protect electrical power supply systems andloads in the event of a short circuit by rapidly building up asufficiently high switching voltage, as a result of which theshort-circuit current is limited, and is interrupted after a short time.In order to increase the current limiting effect, the switching voltagethat is used can be increased by connecting the switching device inseries with a separate current limiter. According to the prior art, thelimiter is for this purpose connected in the main circuit, so that theload current flows through it all the time, both during normal operationand in the event of a short circuit.

[0003] There are various technical solutions for limiters. In additionto conventional mechanical switches which produce switching arcs, PTClimiters are used for current limiting, in which the build up of voltagewhen switching occurs is produced by increasing the electricalresistance of the limiter material and/or by means of a gas dischargewith a high burning voltage.

[0004] In comparison to mechanical switches, PTC limiters have theadvantage that the switching voltage is built up very quickly. Thedisadvantage is the greater cold electrical resistance, as a result ofwhich the rated current must be limited during rated operation in orderto prevent unacceptable heating of the PTC material, for example as aresult of motor starting currents, and unintentional response of thelimiter. In one known commercial product (ABB PROLIM), a circuitbreaker, acting as a switching device, and a PTC limiter arespecifically electrically connected in series in the main circuit.

[0005] Another possible solution for the rated current problem isdescribed in EP 0 657 062 B1, in which a limiter for a circuit breakeris connected in an auxiliary current path, through which current flowsonly briefly, when switching occurs. The auxiliary current path isformed from the arc guide rails and the quenching chamber, and isconnected by the commutation of the arc from the switching contacts ontothe guide rails.

[0006] In comparison to conventional circuits in which a circuit breakerand a limiter are connected in series, there is, however, a risk ofswitching failure, with the switching arc possibly being commutated backto the main current path, when the limiter is connected in the auxiliarycurrent path. When commutation back such as this occurs, the switchingarc or the arc attachment point is moved back from the auxiliary currentpath to the main current path, for example to the switching contacts, asa result of which, although no current passes through the limiter, itdoes, however, generally retain its resistance at that time. If anotherattempt is made for the arc to commutate into the switching chamber, thelimiter switching voltage must then be overcome in addition which, insome circumstances, makes the commutation process so difficult that itcan fail.

[0007] DE 42 43 314 A discloses a current-limiting circuit breaker withan arc quenching device and an auxiliary current path with at least onePTC thermistor and an overvoltage suppressor associated with it. In bothdevices, the switching from the current-limiting mode to thenon-current-limiting mode takes place in a corresponding manner to theovercurrent decay.

[0008] Furthermore, U.S. Pat. No. 5,777,286 discloses an electricalswitching device with separate contacts, which can be disconnectedmechanically, and with an arc switching contact associated with this, inwhich case a PTC thermistor or the like can be connected in theauxiliary circuit. Furthermore, EP 0 350 825 A2 discloses an electricalswitching device with an arc quenching device and a current limitingdevice in the auxiliary circuit.

[0009] Against the background of the latter prior art, the object of theinvention is to specify a method for operation of a switching devicehaving a current limiter in the auxiliary current path, in which thereis no possibility of a switching failure caused by commutation back tothe main current path. A further aim is to provide associatedarrangements.

[0010] According to the invention, the object is achieved by themeasures in patent claim 1. An associated arrangement is specified inclaim 7. The dependent claims contain advantageous developments of themethod and of the associated arrangement. In particular, the arrangementclaims specify advantageous embodiments of the current limiter, whichis, in particular, in the form of a PTC limiter, on the one hand, and/orof the switching device, which is in the form of a circuit breaker, onthe other hand.

[0011] The invention provides in particular the functional reliabilityof a combination specifically comprising a circuit breaker and a limiterin the auxiliary circuit.

[0012] However, the invention is also applicable to other switchingdevices and current limiters.

[0013] Further details and advantages of the invention will becomeevident from the following description of the figures relating toexemplary embodiments, and with reference to the drawing, in conjunctionwith the patent claims. In the figures:

[0014]FIG. 1 shows the combination of a circuit breaker with a currentlimiter in the auxiliary current path of the circuit breaker,

[0015]FIG. 2 shows a current limiter in the form of a PTC limiter,

[0016]FIG. 3 shows the profile of the electrodes and of the resistancebody in the PTC limiter as shown in FIG. 2,

[0017]FIG. 4 shows an illustration, in the form of a graph, of theresistance as a function of time for an arrangement as shown in FIGS. 2and 3,

[0018]FIG. 5 shows a symmetrical circuit breaker with a switching linkand limiter added to it,

[0019]FIG. 6 shows a detail from FIG. 5, with switching chambers in thecircuit breaker, which is in the form of a double-interrupting circuitbreaker, and

[0020]FIG. 7 shows a plan view of the outer guide rails of the switchingchambers, with apertures.

[0021] Parts which are identical or have the same effect have the samereference symbols in the figures. Two measures according to theinvention are essentially described, by means of which, individually orin conjunction, the problem of switching failure of PTC limiters in theauxiliary current path of circuit breakers can be avoided.

[0022]FIG. 1 shows a schematic illustration of the arrangement of alimiter in the auxiliary current path of a circuit breaker, as isdescribed by way of example in EP 0 657 062 B1.

[0023] In FIG. 1, a current-limiting circuit breaker 20 contains atleast two contacts 22 and 23, at least one of which is designed to moveand can be opened and closed via a switching mechanism 24 which can betripped by a thermal and/or magnetic release 25 or 26, respectively.Each contact 22 and 23 has a respectively associated arc guide rail 27and 27′, which surround an initial chamber area 28 and open into aquenching chamber 21 with a large number of splitter plates 29 forquenching an arc, which is not illustrated in FIG. 1. Once the arc footpoints have been struck, the arc moves on the guide rails 27 and 27′into a quenching chamber 17 with splitter plates 29, where an arcvoltage is built up that is sufficiently high for current limiting andarc quenching.

[0024] In the case of very high short-circuit currents, for exampleI_(K)=50 to 100 KA, the increase in the arc voltage is no longersufficient to limit the current flowing through the switch tonon-critical levels. It is then possible for the switching device to bedamaged or destroyed. In order to avoid these undesirable consequences,a limiter 1 is connected upstream of the circuit breaker 20 in theauxiliary current path in FIG. 1.

[0025] The limiter 1 is a current-limiting element which is notconnected in the main current path in a corresponding manner to that inEP 0 657 062 B1, but forms an auxiliary current path in the switchingdevice 20, being passed to an arc guide rail 27. The auxiliary currentpath for a commutation current I_(com) is defined in FIG. 1 as a currentpath in parallel with the main current path for a current i that is tobe switched. Current flows through it when the arc attaches itself tothis guide rail as a result of striking of a foot point.

[0026] The limiter 1 as shown in FIG. 1 is advantageously in the form ofa PTC limiter. A PTC limiter 1 such as this is illustrated schematicallyin FIG. 2 and comprises two planar electrodes 10, between which aresistance body 5 composed of some suitable material is clamped in, witha force K acting on it. The resistance body 5 has surfaces 2 and 3 andthe electrodes 10 have surfaces 11. A PTC limiter such as this forcurrent limiting operates as explained in detail in EP 0 657 062 B1.

[0027] As can be seen from FIG. 3, the flat electrode 10 has profiling15 rather than having a smooth surface 11, and this profiling 15 has asquare-wave structure with a web width b and web height h. The web widthb may be between 0.1 and 1 mm, and the web height h may likewise bebetween 0.1 and 1 mm. In particular, the web width b and the web heighth are of the same order of magnitude, preferably between 0.3 and 0.6 mm.The resistance body 5 has complementary profiling 7 on both surfaces 2and 3. The resistance body 5 and the flat electrodes 10 are connected toone another, such that they cannot be detached, via the profiling 7 and15.

[0028] In an embodiment different to that shown in FIG. 3, the profiling7 and 15 may also be at an inclination angle with respect to the surfaceof the flat electrode. The configuration of the profiling influences theway in which the PTC limiter 1 operates.

[0029]FIG. 4 shows a switching oscillogram for the PTC limiter 1 withprofiling 15 on the electrodes 10 and with a complementary surfaceprofile 7 on the PTC resistance body 5, as is described in detail in EP0 717 876 B1. The time profile of the limiter resistance R when the PTClimiter 1 is interrupting a short circuit can be seen from the curve 17.At the commencement of the short-circuit current, the limiter resistancestarts from its initial value R0≈4 mΩ, and increases slightly. Afterabout 300 μs, it reaches a first plateau level P at about 8 mΩ. Whilethe short-circuit current rises further and reaches the value of 5 kA,500 μs after the start of the short circuit, the resistance curvechanges to a steep rise at this time, and remains for about 300 μs atresistance values which are considerably greater than 100 mΩ. About 900μs after the start of the short circuit, the limiter resistance onceagain falls back to a low resistance value of about 15 mΩ, and thendecreases to its initial value.

[0030] The combination of the characteristics of the arrangementcomprising a circuit breaker and current limiter as shown in FIG. 1 andthe embodiment of the PTC limiter 1 as a current limiter as shown inFIGS. 2 to 4 makes it possible for the limiter 1 to be reset in a timeperiod of a few tenths of a millisecond from the high-resistance state,that is to say the switched state, to the low-resistance state byreducing the current, when an arc is commutated back from the auxiliarycurrent path to the main current path. The additional voltage requiredfor renewed commutation of the switching arc from the main current pathto the auxiliary current path is quantitatively produced by the productof the instantaneous current and the resetting resistance of thelimiter.

[0031] In the example shown in FIG. 4, the resetting resistance is abouttwo to four times the cold resistance. In order not to exceed theadditional commutation voltage of, for example, 50 V with a resettingresistance of about 10 mΩ, the current passing through the short circuitmust therefore not exceed 5 kA. This means that the resetting resistanceof the PTC limiter 1 must therefore be designed to match the magnitudeof the current being passed through it and the maximum commutationvoltage.

[0032]FIG. 5 shows a circuit breaker 20 with a PTC limiter 1 in theauxiliary current path, as is shown in a comparable manner in FIG. 1.The major difference is the symmetrical configuration of the switchingpart 30 of the circuit breaker 20, with a switching link 32 and a doublechamber 21, 28, as well as 21′, 28′, respectively, with respective guiderails 36, 36′ and splitter plates 29, 29′, and with the limiter beingconnected in this symmetrical switching chamber arrangement. Thedesignations of the functional parts correspond essentially to those inFIG. 1.

[0033] As in FIG. 1, the limiter 1 in FIG. 5 is also loaded by thearrangement of the current limiter 1 in the auxiliary current path ofthe circuit breaker 20 only during switching operations. The switchingchamber current path is used as the auxiliary current path and, once theswitching link 32 has been opened, is connected by the arc commutatingfrom the switching link 32 to the adjacent guide rails 29 and 29′.

[0034] In FIG. 5, the current limiter has its own enclosure 50, which isfitted to the enclosure 30 of the circuit breaker 20 and includes anextension 52 for mechanical operation of the switching mechanism 24.

[0035] When the limiter 1 is not connected, then the circuit breaker 20contains, instead of this, a guide rail link 39 for connection of thetwo guide rails 36, 36′. FIG. 5 shows the switching link 32 in theclosed position by means of a solid line, and in the open position bymeans of a dashed line. The current path passes from one of theconnections 47, 47′ into the drive part 40 of the circuit breaker 20,which in turn, as shown in FIG. 1, contains the switching mechanism 24,the overcurrent release 25 and the short-circuit release 26. Inconsequence, when a short circuit occurs, the short-circuit release 26can open the switching link 32 of the circuit breaker 20 without anydelay.

[0036] The limiter 1 is connected to a connection point between the twoguide rails 36 and 36′, which have an associated switching link 32 andare used as arc guide rails. Current does not flow through the limiter 1until the arc attachment has commutated from the link contact to theadjacent guide rail in both switching chambers. The necessity forsimultaneous arc commutation results in the additional voltagerequirement being distributed between the two switching paths, as aresult of the voltage drop across the limiter 1. This splitting effectalso makes it easier for repeated commutation from the main current pathto the auxiliary current path once the arc has commutated back onto theswitching link.

[0037] As a further effect, the double interruption means that theswitching arc cannot move back from the auxiliary current path to themain current path unless this backward movement takes place in bothswitching chambers.

[0038] As a particular measure to prevent arcs from commutatingbackwards in this way, the configuration of the guide rails 36, 36′creates an area which is largely screened from the respective arcsplitter chamber 21 or 21′ and the associated initial chamber area 28 or28′, and this area 34, 34′ holds the link contacts 23, 23′ when theswitching device 20 is in the open position.

[0039] In FIG. 6, an aperture 38 or 38′ has been incorporated in theguide rails 36, 36′, respectively, which are associated with theswitching link 32 being in the open position, and this can clearly beseen in detail in the plan view in FIG. 7. The aperture 38 or 38′ in theguide rails screens the switching link 32 in the open position from thesplitter plates 29, 29′ and from the initial chamber area 28, 28′, thuspreventing arcs from restriking on the switching link 32. This ensuresthat the limiter function does not cease if arcs are restruck in theinitial chamber area 28, 28′.

[0040] As mentioned, FIG. 6 shows the switching link in the openposition, in which the distance between the link contacts 23, 23′ andthe stationary contacts 22, 22′ is considerably greater than thedistance between the guide rails 36, 36′ and the stationary contacts.This result in the arc burning voltage creates a voltage differencewhich assists the arc commutation and makes it harder for the arc tocommutate backwards.

[0041] The screening geometry of the guide rails 36, 36′ prevents arcplasma from being able to flow out of the arc splitter chambers 21, 21′or from the initial chamber areas 28, 28′ directly to the switching link32, and causing flashovers from the switching link 32 to the guide rails36, 36′ or to the stationary contacts 23, 23′.

[0042] As can clearly be seen from FIG. 7, the switching link 32 passesthrough the aperture 38, 38′ in the guide rails 36, 36′ during theopening movement. The chosen geometry results, in a known manner, in amagnetic field being formed, by means of which the arc or arcs is or aredriven onto the cutout edge, and is or are split. The link mount 45 formovement of the switching link 32 is at the same time used forelectrical isolation between the two switching chambers of thedouble-interrupting circuit breaker 20.

[0043] Particularly in the case of the example described with referenceto FIGS. 5 to 7, the circuit breaker is in the form of an individualswitching device with the capability for connection of a limiter. Thelimiter connecting point is for this purpose connected by means of aguide rail link. A mechanical extension 52 for switching the circuitbreaker on and off is provided, if required, for the combination of acircuit breaker and limiter.

[0044] Instead of fitting the limiter to the circuit breaker, ahigh-current version of the circuit breaker can be provided with alimiter that is integrated in the breaker enclosure.

[0045] Examples have been used to show that a circuit breaker isparticularly suitable for the combination according to the invention ofthe switching device with a suitable current limiter. However, acontactor or a semiconductor switch can also be used in a correspondingmanner as the switching device. However, arc switching elements arerequired, in particular for switching without any arcs.

[0046] For the practical implementation of the invention, the switchingdevice and the current limiter can also advantageously include systemengineering means. For example, the current commutation can be improvedby isolating media, such as moving slides, a cover on the main currentpath/contact point. Use with single-interrupting and/ordouble-interrupting contact arrangements has been described. In thiscase, the switching contacts can be provided with a linear openingmovement, or else with a rotary opening movement. Additionally oralternatively, current limiters which have been described in detail withreference to the figures may also be used in the form of a limiter withan additional switching chamber/contact point, or else a solid-statelimiter. Special quick-action releases, for example a piezo-element forswitching to the auxiliary current path at low power levels, can be usedfor early identification of short circuits. Finally, electronic trippingis also possible.

[0047] The described arrangements also allow communication withmonitoring of switching states and/or of the life of the contacts or anindication of the remaining life, as well as an indication of thelimiter life by addition of the short circuits.

1. A method for operation of a switching device, for example a circuitbreaker or contactor, having at least one main current path and anassociated auxiliary current path, with the switching device having aquenching device for an arc, and with a switchable current limiter beingused to switch from the non-current-limiting mode to thecurrent-limiting mode and to form a switching voltage when there is ahigh current load, comprising the following measures: the resetting ofthe current limiter when a change is made from the current-limiting modeto the non-current-limiting mode takes place in a time period of <1 ms,the current limiter is reset when the arc commutates back from theauxiliary current path to the main current path.
 2. The method asclaimed in claim 1, characterized in that the arc at the same timeinterrupts the main current path and connects the auxiliary current pathby commutation from the main current path to the auxiliary current path.3. The method as claimed in claim 1, characterized in that no currentlimiter switching voltage may be overcome when the arc commutates intothe switching chamber.
 4. The method as claimed in one of the precedingclaims, characterized in that a double-interrupting circuit breaker withtwo switching chambers per switching pole is used as the circuitbreaker.
 5. The method as claimed in one of the preceding claims,characterized in that a PTC limiter is used as the current limiter,whose resetting resistance for resetting to the non-current-limitingmode is chosen as a function of the forward current and the maximumcommutation voltage.
 6. The method as claimed in one of the precedingclaims, characterized by the inclusion of the switching device,including the current limiter, in a switchgear assembly having means forcommunication of switching states of the switching device and/orcharacteristics of the switching contacts.
 7. An arrangement forcarrying out the method as claimed in claim 1 or one of claims 2 to 6,having a switching device (20) which has switching contacts (22, 23),arc guide rails (36, 36′) associated with these switching contacts, aswell as a quenching chamber (21), and having a current limiter (1) inthe auxiliary current path of the switching device (20), with thecurrent limiter being a PTC limiter (1) in order to prevent switchingfailure of the current limiter (1) which is arranged in the auxiliarycurrent path of the circuit breaker (20), which PTC limiter (1)comprises a resistance body (5) composed of polymer, which is made to beelectrically conductive, between two metallic electrodes (10) underpressure, and has profiled electrodes (10, 15) and complementary surfaceprofiles (7) in the resistance body (5), and in that the current limiter(1) is connected to a connection point of the two outer guide rails (36,36′) of the circuit breaker (20), where for preventing arcs fromcommutating back, the guide rails (36, 36′) screen the link contacts(23, 23′) from the arc splitter chambers (21, 21′) and from the initialchamber areas (28, 28′).
 8. The arrangement as claimed in claim 7,characterized in that the profile depth of the electrode (10) is between{fraction (1/10)} mm and {fraction (5/10)} mm.
 9. The arrangement asclaimed in claim 7, characterized in that the profile edges are inclinedwith respect to the electrode surface (11).
 10. The arrangement asclaimed in claim 9, characterized in that opposite profile edges of theelectrode are inclined conically with respect to the electrode surface.11. The arrangement as claimed in claim 7, characterized in that currentflows through the current limiter (1) when the arc attachment iscommutated from the link contact (23, 23′) to the adjacent guide rail(36, 36′) in both switching chambers of the circuit breaker (20). 12.The arrangement as claimed in claim 7, characterized in that theswitching link (32) of the circuit breaker (20) is designed such thatthe distance between the link contact (23, 23′) and the stationarycontact (22, 22′) when the switching link (32) is in the open positionis greater than the distance between the guide rail (36, 36′) and thestationary contact (22, 22′).
 13. The arrangement as claimed in claim12, characterized in that the screening geometry of the guide rails (36,36′) prevents arc plasma from flowing from the arc splitter chamber (21,21′) or from the initial chamber area (28, 28′) to the switching link(32).
 14. The arrangement as claimed in one of claims 10 to 13,characterized in that the link mount (45) for movement of the switchinglink (32) is also used for electrical isolation between the twoswitching chambers (21, 21′) of the circuit breaker (20).
 15. Thearrangement as claimed in one of claims 10 to 13, characterized in thatthe switching device is in the form of a single switching device (20)with the capability for the current limiter (1) to be connected.
 16. Thearrangement as claimed in claim 15, characterized in that the connectingpoint of the current limiter (1) to the switching device (20) isconnected by means of a guide rail link (39).
 17. The arrangement asclaimed in claim 16, characterized in that, when the current limiter (1)is connected in the auxiliary circuit, the guide rail link (39) isreplaced by the current limiter (1).
 18. The arrangement as claimed inclaim 17, characterized in that the current limiter (1) is arranged inits own enclosure (50).
 19. The arrangement as claimed in claim 18,characterized in that the current limiter enclosure (50) can be fittedto the switching device enclosure (20).
 20. The arrangement as claimedin claim 19, characterized in that a mechanical extension (52) forswitching the switching device (20) on and off is provided in theenclosure (50) of the current limiter (1).